None.
Tire uniformity machines, in general and their use and function are well known in the art. As part of an overall testing and inspection program conducted in connection with the production of pneumatic tires, these machines are utilized to take various measurements characterizing the uniformity of a tire. This characterization is accomplished by measuring the nature of forces generated by the tire as it rolls or is rotated along a surface.
In general, the typical tire uniformity machine includes a frame and associated conveyors moving tires to be tested into and out of the frame. Inflation means and a chucking assembly are provided to inflate the tire and rotate the tire against a loadwheel which is selectively movable into and out of engagement with the periphery of the tire. Various sensors and other pieces of instrumentation are connected to the loadwheel to obtain the desired information and measurements concerning the reaction characteristics of the tire. SAE specification J332, to which most tire uniformity machines are manufactured, specifies that the machine""s loadwheel have a diameter of 33.625 inches with a mean radial force equal to 85% of the tire and rim association load rating for the tire.
In a typical loadwheel assembly, the loadwheel is rotatably mounted on a carriage supported by a pair of L-shaped gibs or xe2x80x9cTHKxe2x80x9d style rails allowing the loadwheel to be selectively brought into and out of contact with the tire for purposes of simulation. In one known system, the motion of the carriage is controlled by a D.C. motor via a 15:1 gear box reduction and a 54Txe2x88x9223T chain-sprocket drive increase with a resulting reduction of 6.39:1 to a 0.25 inch pitch ball screw. The maximum speed of such a system is about 1.1 inches per second. Loading is accomplished by using a 4-quadrant D.C. drive that controls the velocity of the loadwheel carriage. Using the velocity control method, the loadwheel is initially advanced at full speed until its load cells detect increase in the instantaneous load. When the measured load is within 200 pounds of a setpoint load the analog velocity setpoint to the D.C. drive is reduced as the xe2x80x9cload errorxe2x80x9d decreases. In this system, it is necessary to load to an average radial force, which is calculated over one or more revolutions of the tire.
A typical load cycle for such a system consists of the following events and time periods. The loadwheel is advanced from a retracted position to the face of the tire at the maximum velocity of about 1.1 inches per second to a distance at least 1 inch from the tire to ensure clearance during tire chucking. This event typically takes 0.9 seconds. From this point, the loadwheel is advanced from tire contact to within 200 pounds of a setpoint load at 1.1 inches per second. This event covers a distance of about 1 inch based on a load setpoint of 1200 pounds and a tire spring rate of 1000 pounds per inch. The event takes about 1 second to perform. Finally, the loadwheel is advanced to an average setpoint maintaining an error window of plus or minus 20 pounds of the load setpoint and at a velocity proportional to the average load error. The loadwheel moves approximately 0.2 inches and requires 1.5 to 2 seconds for this event. Overall, the total distance moved by the loadwheel is about 2.2 inches and takes about 3.4 to 3.9 seconds to perform. Actually loading time or xe2x80x9cservoxe2x80x9d time is often much longer due to servo tuning problems that result in overshooting or undershooting the desired load setpoint. A large variation in load setpoints and tire spring rates adds to the difficulty of achieving a precise load setpoint regardless of cycle time. These difficulties have been described in U.S. Pat. No. 4,704,900 to Beebe, which uses a plurality of instantaneous radial force measurements with a corresponding instantaneous loadwheel position measurement to control the measurement to control the commanded position of the loadwheel.
It is, therefore, an aspect of the present invention to provide a more accurate and rapid system for imposing the required average force on a tire by a loadwheel.
The present invention generally provides an apparatus and method for accurately and rapidly imposing the required average force on the tire by using a drive mechanism, which initially drives the loadwheel carriage to a known average load position for tires of the type under test. This average can be computed based on a single previous test tire or based on an entire population of previously tested tires of the same or similar type.